The present invention relates to a laminate structure comprising a protective layer and an exposed photopolymer layer, and to a process for producing the laminate structure. Photopolymer layers of the type mentioned at the outset for production of holographic media are known from WO 2011/054797 and WO 2011/067057. Advantages of these holographic media is the high light diffraction efficiency thereof and that no subsequent processing steps are needed after holographic illumination, for example chemical or thermal development steps.
DE 699 37 920 T2 states that holographic photopolymer layers can change colour when substances swell into the photopolymer layer from adjacent layers, such as adhesive layers, or bleed out of this layer into the adjacent layer. If one of the two phenomena occurs, there may be an expansion in volume or a shrinkage in volume in the photopolymer layer. This in turn leads to a long-wave or short-wave colour shift in the hologram. Especially in the case of multicolour holograms, this brings about unwanted visual colour changes.
In order to avoid changes in volume and the associated changes in colour, DE 699 37 920 T2 teaches adding sufficient amounts of the swelling or bleeding substances to the adjacent layers and/or to the photopolymer layer beforehand. This process, however, is costly and inconvenient. Furthermore, according to which material is to be used for the adjacent layer, an adjustment has to be made. Finally, the added substance must also be selected such that it does not destroy the photopolymer layer.
It was therefore an object of the present invention to provide a laminate structure comprising a protective layer and an exposed photopolymer layer, which can be produced in a simple manner and can be bonded to a wide variety of different adjacent layers, for example adhesive layers, without resulting in any changes in volume of the photopolymer layer and therefore any associated changes in the colour of the hologram.
This object is achieved by a laminate structure comprising a protective layer and an exposed photopolymer layer, the laminate structure being obtainable by reacting at least one radiation-curing resin I), an isocyanate-functional resin II) and a photoinitiator system III), and the radiation-curing resin I) containing ≦5% by weight of compounds having a weight-average molecular weight of <500 and ≧75% by weight of compounds having a weight-average molecular weight of >1000, the isocyanate-functional resin II) containing ≦5% by weight of compounds having a weight-average molecular weight of <500, and the protective layer containing the radiation-curing resin I) at least to an extent of 80% by weight and the isocyanate-functional resin II) at most to an extent of 15% by weight.